Most of us will agree that San Guo Yan Yi paints a rather disoriented picture of who Liu Bei really was. Instead of focusing on his ability to rule, Luo Guanzhong focused on the moral aspect of Liu Bei's legitimacy. For instance there were large accords of his link back to the Prince of Zhongsheng, his endless crying about taking land from relatives (Liu Biao, Liu Zhang) and of course all the reference to Liu Bang and other Han people.
However, because of this, there is less information on what Liu Bei did do that made him a good ruler.
Liu Bei pretty much came from the least favourable background of the three Kings. Cao Cao had the luxury of his father and the benefit of his family's fame. He was not as good as most cosider him to be, simply because he had his share of losses, and because it was mostly other people that attributed to his victories.
Sun Quan of course already had Jiang Dong when he started, though merely a child, he already owned power.
Liu Bei was a nobody with no experience who managed to get from nothern YouZhou to southern YiZhou, defeated many opponents along the way. Zhuge Liang historically didn't participate in fighting as Liu Bei did, who even defeated an asteemed general like Xiahou Dun, without help.
The biggest problem for Liu Bei in fact became the few things that he had going for him. His legitimacy with the Han made his decisions against Liu Zhang and others seem treacherous, simply because it made him look like a real ruler. Just as Cao Cao and Sun Quan, Liu Bei had to take advantage of the situations around him.
Forming Shu in between Wei and Wu, two powerful nations, and dodging assassination attempts by people like Cai Mao, made it quite an acheivement.
I don't care much for the fictional Liu Bei, but the real Liu Bei deserves more credit.
I don't think people like Fa Zheng, Ma Liang and Fei Wei, whom I hold in high regard, would have joined Liu Bei if he had no potential.
Propriety and moral aside, Liu Bei as an effective ruler who was ruined because of the loss of Jingzhou and his own mistake at Yiling.
Should he been given the chance to fight Sima Yi, I don't doubt he could have advanced more into the north.
Internal strife and foreign relations caused the doom of Shu, with Liu Bei, the dead Emperor as blame.
He who exercises government by means of his virtue may be compared to the north polar star, which keeps its place and all the stars turn towards it.